Subtitles are words used in video (TV and movies, video games) that give the dialogue spoken in the video, and descriptions of other sound. For our purposes, these can be split into parallel English dialogue with L1 subtitles, parallel L1 dialogue with English subtitles, and finally non-parallel English dialogue with English subtitles.
L1 subtitles can help students understand the meaning of the video, however learners often default back to L1 thinking, which is disastrous. Note however that often the translation is imperfect, and might not adequately convey meaning. Turning off or covering subtitles can help students focus on the spoken English.
English subtitles can help students to follow English dialogue. Unfortunately however subtitles often inaccurately reflect the actual dialogue spoken, or contain other errors.
In some countries, e.g. Netherlands and Portugal, English TV is subtitled whereas in other countries, e.g. France or Spain, English is usually dubbed. There is evidence (at least anecdotally), that people tend to dislike subtitles, and choose to ignore them, which gives them practice at English listening and comprehensible input. This may be a main causal factor why Portuguese are generally better than Spanish at English, other factors such as linguistic distance and culture being roughly equal.