The zombies, being as (brain)dead as they are, only attack in a straight line, which gives you a chance to mount, and plant, a defensive line. Being the green fingered sort, it's up to you to plant a variety of different flowers, in an attempt to hold off, and destroy the zombie hordes. It's a very rock/paper/scissors affair, with certain flowers being more effective against certain zombie types, meaning getting the balance right is important, as just one hole in your defensive wall will let the zombies through. Whether you're planting sunflowers, which produce the game's currency, sun points; adding a new double pea shooter, which fires peas at the undead hordes; or planting a cactus, to shoot down the dastardly balloon zombies, which attempt to float over your defences towards your house, there's a lot of forward planning required here, as this is a game that you need to think through.
In terms of accessibility, though, Plants vs Zombies is as easy to pick up and play as a game like this can be. The idea behind the game is introduced slowly and clearly, with each new item - or zombie - type getting their own intro and/or description explaining how it works. On the other hand, there's nothing in the way of voice acting here, and lots of text to read, which is essential to understand if you want to be able to defeat the hordes - although that said, it's usually pretty obvious what each item does once you've plonked it down.