It is part of the tree…a knot, branch, etc.
Nothing to worry about.
Like Larry said.
Looks like #3 common framing lumber or #4 common.
Not a good grade lumber for Fiber Stress loading. But won’t go anywhere. 
Not a defect as far as i am concerned, if no deflection, cracking, etc. is observed.
big azz knot as others have said…common…
Knots can indeed present a structural problem depending on the size and spans that are used however in this case the purlins below make it a moot point.
Jeff