What is something you say to your pet that he or she truly understands?

Tuna.

Any sound I make will draw in my pet cats from the woods.  

Who is my handsome oobly boobly baby boy?  It makes him roll over and show me his ample and hairy tummy. 

They know their name, come here, good girl, good boy, don't, no, what is that kitty doing? (they almost always look at the cat I'm talking about), mousie, birdie.  Mousie is what I call a chipmunk that frequents my back door. This post was edited by Art Lover at August 20, 2019 9:42 PM MDT

Recent research shows that animals (pets) understand not just words, but tone.
They get stressed when the words do not match the tone.

EG: If you say "good boy" in an angry tone.

When he is up and about I say 'treat!' my cat will head toward the kitchen and wait for me to dig out a spoonful of yogurt for him to lap up.

My dog understands a few words and there is also a hand signal for ever word he knows. Like if I say "now" he knows to start a perimeter check or I could make a fist then open it with my fingers widely spread and he knows to do a perimeter check. Cheers!

Beyond the basic commands of Sit, heel, stay and protect the phrase that gets them excited is "let's go".  they will go and sit at the door until their leashes are on

My cat understands "no", "food", "hello" and "sorry".
However, I cannot be sure that she really understands the words. 
I think it's more likely that she reads them in context with the situation, my tone of voice and my facial experessions.
She's unerring at reading my moods.

My horses understand: 
come sweetie = come here/follow me
stay = stay put here
click-click = move faster
slow-leeee = move slower
back-back = move backwards
move over = shift sideways away from me
lift = pick up this hoof
walk
trot
canter
trot
woah (said with a slow drawl) = stop
good boy/girl = well done

I have birds.  They understand TWITTER.  

They, like me, HATE Trump and how he has ruined Twitter with his horrible Tweets.  My birds understand birdbrains.  And they tell me in their tweets that he has no brain.  At all.  

My birds read his Tweets and they tweet back LIAR LIAR LIAR.

They are stable. And they are geniuses.

This post was edited by WM BARR . =ABSOLUTE TRASH at August 21, 2019 6:06 PM MDT

My old dog seemed to understand a lot. I talked to her like she was a human and she seemed to understand the context or the meaning at least 75% of the time. I could ask her things like, "Why are you in the way?" and she'd move. She was crazy smart. 

My son's girlfriend's dog understood various words for relieving himself. He was weird. We could take him outside for a potty break and he wouldn't go, but if you told him to pee or poop, he'd go do it on command. 

My new dog is just barely starting to catch onto some basic things. We've had him for maybe three weeks now. The pound listed him as being two, but I think he can't be much more than one. He did not listen much at all when we got him, but now he's responding to basic commands like "sit" and "stay." I think he understands the difference between various toys now too. He understood "ball" early and would go get a ball on command. Now he'll get "squeaky" when we ask him to as well. And, he seems to understand "give," which is the command I use when we're playing fetch or he's picked something up from around the house he shouldn't have. He doesn't always listen when he has a toy and wants to keep playing solo, but he's pretty good about it the rest of the time. 

He's starting to get the hang of "leave my cat alone" as well, but it could be the tone. lol He's still got a lot of bad habits I need to break him of, like snatching food off the counter. If I see him snooping, I'll ask, "What do you think you're doing?" and now he'll stop what he's doing, make sad faces, and go sit down. I think he'll probably wind up being on par with my last dog, but it's hard to say because he's still got a lot of puppy left in him. 

I don't have a dog but when you ask my sister's dogs if they want a treat, or to go for a walk, or a ride, they come flying.

My cat knows nothing. He will lokk at me when I ask "what do you think you're doing?" But will continue whatever it is. 

My long haired doxie knows come, lay down, do you want go outside?, go on now (leave the people alone and go lay down). She "knocks" on my son's door until he lets her in.