The word "sec" is a short form of the word "second." It is commonly used as an abbreviation to represent a unit of time measurement that is equal to 1/60th of a minute. Full definition
Think about this for sec... could you imagine if doctors got paid by how fast they performed surgeries or Police officers by how many tickets they wrote. (humblemechanic.com)
You can def leave it out or use triple sec or something of the like. (howsweeteats.com)
Always focus on your large muscle groups, doing 8 - 12 reps with 60 sec rest between your sets, intensity is key when building muscle. (fitness-baron.com)