You've hopefully already learned that 力 means "power". You might have also heard the word いっぱい, because it's used a lot in conversation. It has a few meanings (you'll learn more of them later) but the important one here is "full" or "a lot". Put these two words together and you have full power, with all one’s strength, and as hard as one can. Remember this one with all your strength!

Just a side note: いっぱい can also be written in kanji, which you'll learn later on! However, in simple words like this one it's usually written in kana.