Hey, äta is the infinitive form (to eat), while äter is the conjugated form (I eat, you eat, he eats etc..) Jag äter ett äpple - I am eating an apple. Jag vill äta ett äpple. - I want to eat an apple.

"Äter" is something you do right now, like "eating". "Äta" is the primary form and you can also say "att äta", which means "to eat". So yes, it matters. The second one sounds very wrong and need another word between "jag" and "äta". For example "ska" or "vill".