sich treffen
to meet
To come together with someone, either by arrangement or by chance.
Ich treffe mich heute Abend mit Freunden im Kino.
I'm meeting friends at the cinema this evening.

to meet / to hit
Loading your saved state.
to meet
To come together with someone, either by arrangement or by chance.
Ich treffe mich heute Abend mit Freunden im Kino.
I'm meeting friends at the cinema this evening.

It is important to distinguish between 'sich treffen' (to meet) and 'treffen' (to hit/affect). 'Sich treffen' means that two or more people meet. 'Jemanden/etwas treffen' means to hit a target or to affect someone.
Ich treffe mich heute mit einem Freund.
I'm meeting a friend today.
When arranging to meet with a person, you use the preposition 'mit' (+ dative). The place of the meeting is often given with 'an' or 'in' (+ dative).
Wir treffen uns morgen mit den Kollegen.
We're meeting with the colleagues tomorrow.
'Treffen' is part of many fixed noun-verb collocations. The most common are 'eine Entscheidung treffen' (to make a decision), 'Vorbereitungen treffen' (to make preparations), and 'eine Vereinbarung treffen' (to make an agreement).
Wir müssen für die Party noch einige Vorbereitungen treffen.
We still have to make some preparations for the party.
In a figurative sense, 'treffen' (to affect/hit) can touch someone emotionally, usually negatively. It is often combined with adverbs such as 'tief' (deeply), 'hart' (hard), or 'persönlich' (personally).
Seine unfreundlichen Worte haben sie sehr getroffen.
Her unfriendly words affected her deeply.
The impersonal expression 'Es trifft sich gut/schlecht, dass ...' (It is convenient/inconvenient that ...) is used to say that a time or situation is suitable or unsuitable.
Es trifft sich gut, dass du da bist, du kannst mir helfen.
It's convenient that you're here; you can help me.
In the literal sense, 'treffen' means to hit a target, for example with a ball, arrow, or shot. A well-known idiom for this is 'ins Schwarze treffen' (to hit the bullseye).
Die Bogenschützin hat mit jedem Pfeil ins Schwarze getroffen.
The archer hit the bullseye with every arrow.
'Treffen' can also mean that you guess something correctly or capture the most important point of a matter exactly. A very common idiom is 'den Nagel auf den Kopf treffen' (to hit the nail on the head).
Mit deiner Bemerkung hast du den Nagel auf den Kopf getroffen.
With your remark, you hit the nail on the head.
to see
'sich sehen' and 'sich treffen' can both mean that people come together. 'sich treffen' more strongly emphasizes the specific meeting, often with a place, time, or arrangement. 'sich sehen' often sounds a bit more general and personal: for example, you say that you have not seen someone for a long time, even if what you actually mean is that you have not met up. The words are easy to confuse because in everyday language people often say 'Wir sehen uns' (we'll see each other) when they mean a meeting. If you want to emphasize the arrangement or appointment, 'sich treffen' is more precise.

Am Wochenende treffe ich mich mit meiner Kollegin zum Kaffee.
At the weekend, I'm meeting my colleague for coffee.
Wir haben uns seit dem Sommer nicht mehr gesehen.
We haven't seen each other since the summer.