# we are surrounded by such a large cloud of witnesses

"there are so many examples of faithful people about whom we learn in the scriptures" 

# witnesses

Here "witnesses" refers to the Old Testament believers in chapter 11 who lived before the race of faith that believers now run.

# let us lay aside every weight and easily entangling sin

Here "weight" and "easily entangling sin" are spoken of as if a person could take them off himself and put them down. 

# every weight

Attitudes or habits that keep believers from trusting and obeying God are spoken of as if they were loads that would make it difficult for a person to run while carrying. 

# easily entangling sin

"sin that makes obeying God difficult"

# Let us run with perseverance the race that is placed before us

"Let us continue obeying what God has commanded us, just like a runner keeps going until the race is over"