"Let's do this!" Is an informal phrase that expresses excitement about an activity. We say it when there is a large task to accomplish. For example, during the US elections many people who support Biden for president posted the phrase, "Come on, people. Let's do this!" To mean, everyone needs to vote for Biden. It indicates that there is a difficulty in the task ahead but that you express enthusiasm to do it. It is slang. And informal. Don't say it to your boss.

هيا نفعل هذا