Teching refers to the process from basic Level 1 factories, units, and ecomony structures to more capable (and expensive) Level 2 (or Level 3, depending on the mod) alternatives. Teching at the right pace is vital to victory. Attempting to tech too early results in an unmanageable drain on resources, often preventing a player from effectively responding to enemy attacks. Teching too late means one is liable to be out-produced by the enemy's superior economy and out-fought by their superior units.
The right time to tech is as soon as your level 1 economy's on solid footing. You want enough combat units and fixed defences to protect your base against enemy assaults and conduct a few raids. Exactly when this point is will vary from map to map. If your enemy hasn't been raiding you, you might be able to put off teching a little longer... Or might be forced to do it earlier, as he's been spending less on combat units.
Once one L2 factory has been established, the first priority is generally to build Advanced Construction Units, to begin construction of a L2 economy (Moho Mines, Fusions, etc) and defences. During and after this, construction of L2 combat units can begin, along with construction of other L2 factories to diversify one's forces.