With All One's Strength, Full Power, As Hard As One Can
adverb, expression
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.
ちからいっぱい
力 uses the same reading as you learned with the vocabulary. That's because this is basically two separate words stuck together.
力いっぱいがんばりました！
We did our best!
ドアを力いっぱいおしてください。
Please push on the door as hard as you can.
リレーでは力いっぱいはしりました。
I ran as fast as I could in the relay.