"どうも (Domo)" has several meanings.
- Hi/ Hello/ Hey
- Thank you
- Somehow
If you use it without combining with other words and just say "どうも (Domo)", it usually means Hi/ Hello/ Hey and is a casual greeting between close friends. If you use it after someone does something for you, or when talking about something nice someone did for you, it's a short form of expressing your appreciation. (Kind of like the abbreviated version of "Domo Arigato Gozaimasu".)
However, when you use it in within a sentence, it usually has this nuance of "somehow~" or "I'm not sure but~" or "it seems to be~", which shows the situation of something which is vague and not so clear.
It's a little bit difficult to use "どうも (Domo)" in a sentence, so if you are a beginner of the Japanese language, I recommend for you to just use "どうも (Domo)" to a new Japanese friend when you want to greet him/her to simply say "Hi!".
