‘-겠-’ convey supposition, a guess, a thought or observation. It can sometimes be translated as ‘I think’, ‘I bet’, ‘I guess’, ‘It/this looks ~’, ‘It must be ~’. This pattern can not be used to express the thoughts or supposition (etc) of a second or third person and must express the thought of the person saying or writing it.
● 내일은 비가 오겠어요. (I bet it’s going to rain tomorrow.)
● 맛있겠다! (Looks delicious!)