It is actually two words. Man is Sanskrit for "the thinking mind." Tra is Sanskrit for "to liberate." When we use a mantra, we are freeing ourselves from our thinking mind so that we can go deeper into our consciousness. Take a moment right now to notice your breath in your stomach or gut. From that place in your stomach ask your thinking mind, "what do you need to hear today?"
The answer I got was, "nothing bad is going to happen." So that's my mantra for the day. If I feel myself getting anxious or start to spiral, I'll give my thinking mind this little phrase it needs to chill out.
ps. This is the translation I was given from a Nepalese Buddhist monk. There are other translations for this word out there.