OPT means Optional Practical Training. It is temporary employment that allows international students to work in the USA and is directly related to your F-1 major area of study. This certainly helps you get practical life experience. Once your details are verified and eligibility criteria are met, you can easily obtain OPT employment authorization for up to 12 months. Also, students pursuing STEM(Science, Technology, Engineering, and Maths) subjects can extend this further for 24 months. Generally, students need OPT once they graduate, but you can apply for it even when you are studying!
However, in pre-completion OPT, the duration will be deducted from the overall period.