集中(jí zhōng) - to concentrate, to focus, to gather together in Mandarin

集中 (jí zhōng) is both the physical act of gathering things into one place and the mental act of concentrating. Same word for stacking all the boxes in one corner and for getting your head down to revise.

For attention it wants an object: 集中注意力 (jí zhōng zhù yì lì) or 集中精力 (jí zhōng jīng lì). Chinese does not really let you say "I am concentrating" with nothing after the verb, so learn it as a two-part chunk and you will sound far more natural.

Watch the tones: jí is second, zhōng is first, and the second syllable is the flat 中 you already know from 中国, not the falling zhòng of 重要. Say it with a falling second syllable and you have wandered into a different word entirely.