There is a series of prefixes to the verb root. As previously mentioned the prefix used depends on the first letter of the verb.
me: l, m, r, w, y
men: c, d, z, t (the 't' is dropped from the root, for e.g. tulis = write; menulis = to write)
mem: b, f, p (the 'p' is sometimes dropped from the root)
meny: s (the 's' is dropped from the root)
meng: g, h, k, a, e, i, o, u
In Indonesian, there are lots of prefixes that we can add to the words, but the most common prefix that we will see is Indonesian me- prefix.
Prefix me- is used to form active verbs. Prefixing any words with me- would require a minor spelling change/inflection in order to facilitate a smooth transition in pronunciation. The inflection is based solely on the first letter of the original word.
As for the meaning, it's exactly the same as the infinitive. In daily conversation, people often drop the me- prefix.
Thanks @demoksaputra and @deantt1.
What is an 'active' verb as opposed to just a normal verb?
If we can omit the prefiix me- verbally, do we tell the difference by the context?
I suppose you suggest that "Saya tulis surat" is correct verbally.