Xjyuk

While playing Baba is You, when I push a word block (such as PUSH or OPEN) into an active position (where it forms part of an active rule like FLAG IS OPEN), it sometimes lights up: the color of the block becomes brighter. However, this doesn't always happen. In the gif below, you can see that FLAG IS PUSH is active (since I pushing the flag), but PUSH is not lit up. However, once OPEN is pushed in, making the rule FLAG IF OPEN, the OPEN block does light up!

What causes blocks to light up in Baba is You? What does it indicate?

All blocks light up whenever you line them up into any valid sentence.

In the image in the question, PUSH is lit up when "FLAG IS PUSH" is lined up, and "OPEN" is lit up when "FLAG IS OPEN" is lined up. It is hard to tell that PUSH is lighting up because you are moving the blocks, and because PUSH's unlit state looks a lot like its lit state, and because OPEN's color when unlit looks a lot like PUSH's color when lit. If you crop out just the PUSH block from each image you will see that they are in fact different colors.

In the game, the difference in the word "PUSH" when it is active vs inactive is pretty subtle, and is easiest to see if you're not moving it. In this animation I think the difference is mostly getting obscured by the fact that OPEN is going active at the same time PUSH is going inactive, and the fact that OPEN's inactive color looks a lot like PUSH's active color. – murgatroid99 in a comment

though i haven't bought nor played the game, based on videos and the trailers, it seems that they light up when a valid action is made; for example, in the second trailer featured on the Steam page, the four poem stanzas were alight, up until the player pushed from the bottom and "Baba" was no longer lit, due to no longer having anything to end it.