It is a little difficult to explain in English because when you use translation tools, you’ll get results like ‘effort’ or ‘endeavor’.However, this phrase means ‘tenacious attitude/personality’.
努(do)= This character carries many definitions such as ‘effort’, ‘exerting effort’, ‘trying’, ‘working towards something’, etc. It really depends on how you use it in a context.
力(ryo-ku)= It literally means ‘power’. This is not limited to ‘muscle power’ but can also be associated with ‘abilities’, ‘skills’, ‘economic power’, etc.
But when used together,
努力= To keep ‘striving hard’ despite setbacks or obstacles. This gives an image of a very tenacious person exerting tremendous effort despite setbacks or obstacles. This is so far the closest definition. We usually say “I/You should 努力.” in many situations and it is widely used in Japan.
If you are interested in these Kanji, you may simply copy them from this website and use them as you please. Make them yours!