试镜 (shìjìng) means "audition" or "screen test." It combines 试 (to try/test) with 镜 (mirror/lens, referring to a camera lens). It specifically refers to the process of auditioning for a role in film, television, or theater. It can be used as both a noun and a verb.
她明天要去试镜一个电视剧的女主角。
Tā míngtiān yào qù shìjìng yí gè diànshìjù de nǚ zhǔjué.
She's going to audition for the lead actress role in a TV drama tomorrow.
试镜的时候他特别紧张,台词差点忘了。
Shìjìng de shíhou tā tèbié jǐnzhāng, táicí chàdiǎn wàng le.
He was especially nervous during the audition and almost forgot his lines.
导演看了上百个人的试镜,最后选了她。
Dǎoyǎn kàn le shàng bǎi gè rén de shìjìng, zuìhòu xuǎn le tā.
The director watched auditions from over a hundred people and finally chose her.