This is the perfective past tense. You can tell because the typical past tense ending of a verb is -л / -ла / -ло (m / f / n respectively) (usually you just chop the infinitive ending, such as the -ть, and then just toss the past tense ending on).
"They meet me at the park" would be Они встречают меня в парке. It is the imperfective present tense of the verb "To meet".
Встретить = When a person (group, w/e) meets another person (group, etc.).
In that case, it is: (Nominative subject - who is doing the meeting) встретить (conjugated for subject) (accusative object - who is being met)
Like in this sentence - Они встретили меня - They met me.
Встретиться has that reflexive -ся (-сь) ending, which indicates that the action was reflected/done back onto/shared jointly with the person/group doing it. In that case, it's just... (Nominative subject) встретиться (conjugated for the subject).
For example - Мы встретились в парке - We met in the park.
Hopefully this makes sense, I'm happy to clarify/give further examples if necessary!