只是 (zhǐ shì) has two uses: as an adverb meaning 'merely, just' (softening or downplaying), and as a conjunction meaning 'but, however' (mild contrast). As a conjunction, it's softer and more conversational than 但是 (dàn shì) or 可是 (kě shì), which express stronger contrast. As an adverb, it's similar to 不过 (bù guò) 'only, merely' but 只是 is more common in everyday speech.