synchro summon
A synchro summon is a method of summoning monsters in Yu-Gi-Oh!.
Introduced in Yu-Gi-Oh! 5D's, a synchro summon combines a "tuner" monster with another non-tuner monster to form a "synchro" monster. For example, if you control a level-4 Tuner monster and a level 4 non-Tuner monster, you can combine them to form a level-8 Synchro monster. Only 1 Tuner monster can be used for a single Synchro summon, but multiple non-Tuner monsters may be used.
In the anime, you will notice a green ring surrounding the non-Tuner monster(s). The synchro monster is then formed.