A microweld is an area where the microscopic bumps on two surfaces stick together. Microwelds are the source of friction.
Glues are generally not recommended for repairing thermoplastic olefins like the TPO, TEO and polypropylenes car bumpers are made out of. Virtually no adhesive will permanently bond a bumper with any sort of durability. The most cost effective way to repair a thermoplastic bumper is to use something like the Urethane Supply Company's MicroWeld 80 airless plastic welder. They are inexpensive and work VERY well, bonding the two parts back together. The process is simple. 1) Clean the part with plastic cleaner 2) V-groove the crack with a dremel tool or something similar. 3) Sand the area BY HAND with 40 grit sand paper. 4) Fill the groove with FiberFlex Universsal welding rod 5) Sink a piece of reinforcing mesh into the back of the repair and add some more FiberFlex 6) Sand the Bumper smooth on the front up to 600 grit 7) Spray on some Adhesion Promoter (1050 Plastic Magic works GREAT!) 8) Spray a primer, then your color coats of paint. In the end you will save a lot of money and the repair will make your bumper as good as new!