A co-write is a process in which two or more songwriters collaborate on writing a song. Generally, each writer gets an equal amount of credit for their effort (2 people, 50% each; 3 people, 33% each, etc.).
Co-writes usually consist of about 20% talking, 10% creating and 70% editing, often in that order. The 20% talking portion allows the songwriters to catch up on life, effectively getting on the same page. Sometimes an idea for the song will be born out of these conversations. The 10% creating portion is where the writers share from their notebooks of potential song ideas, or they fiddle around on instruments and create something out of nothing.
When the writers get inspired by the same melody, lyric or chord progression, the song has begun, and continues developing in a type of ping-pong-style conversation. This can be the most nerve-wracking and intimidating part of collaborating creatively with someone else, but I find it the most exciting and energizing at the same time.
As mentioned before, editing is 70% of the co-writing process. This is where a song goes from good to great. This is where the music and lyric are married, creating an inseparable bond. For more on the editing process, read my previous post here.