Bright
Cheerful
い adjective
This is a single kanji plus the hiragana るい. Words that end in い are usually adjectives, and this is no exception. What's the adjective form of bright? It's also bright.
あかるい
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You have to remember the あか portion. What is あか in Japanese? It is "red." So, think of something that's really bright. Now, imagine it being totally red (aka あか). In Japan, the color red is associated with the sun, and the sun is pretty bright. If you can remember this you can also remember the reading of this word.
このへやは明るい。
This room is bright.
トーフグのみんなは明るい。
Everyone at Tofugu is cheerful.
未来はまだまだ明るいよ、おじいちゃん。
Your future is still bright, Grandpa.