What the research says

Meditation has been oversold as well as undersold. Here is a plain summary of what good studies have found.

Meditation

Modest help with anxiety, low mood and pain. A large review of trials found mindfulness programmes helped with these about as much as other active treatments, and not more. Goyal and others (2014), JAMA Internal Medicine.
Fewer relapses of depression. Mindfulness-based cognitive therapy (MBCT) is recommended by NICE to help prevent depression coming back, for people who are better now but at higher risk of it returning. NICE guideline NG222.
Not for everyone. A minority of people have unpleasant effects. See If it feels worse.

Calming, quickly. Reviews link slow breathing (under ten breaths a minute) with feeling calmer and more relaxed. Zaccaro and others (2018).
Five minutes a day helps mood. In one month-long study, five minutes a day of slow breathing with long out-breaths ("cyclic sighing") improved mood more than five minutes of mindfulness. Balban and others (2023), Cell Reports Medicine.

It doesn't treat any illness, and it isn't therapy.
It won't fix things that need fixing in your life, like money, housing, work or relationships.
If you are struggling, a practice can sit alongside proper help, not replace it. Where to find help.