A mantra is a specific word or phrase that is chanted (or spoken) over and over. Mentally, or aloud.
They can be difficult to traslate, but its not really needed to know exactly what they mean.
Mantras sound powerful. The construction of the words means they resonate with your subtle body.
The most well know mantra is 'Om'.
It is a primal sound, starting at the very beginning of time.
Another popular mantra is 'Om mani padme hum', which translated says "hail to the jewel of the lotus'.
The 'jewel' being the divine spirit and the 'lotus' of our being.
You can create your own! Something like 'love' or 'harmony' will suffice.
You can even incorperate your own religous beliefs into this.
Mantra's can seem embarrasing at first. Once you get over it, you will find out their benefits. It is a very nice form of meditation.