A robo advisor is an online platform that gives you automated investments advice. You provide your risk appetite and based on this the robo advisor recommends you an investment portfolio. These portfolios are actively managed, which means the robo-advisor changes the elements of the portfolio.
Everything is automated, so after signing up you don't have to do anything, just transferring in your savings. With robo-advisors you get convenience but less control on your portfolio. It also cheaper then mutual fund investing since everything is automated, thus cost-effective.
The biggest robo-advisor in the US are Wealthfront, Betterment and Vanguard. In the UK and Europe, Nutmeg and Scalable Capital are the leading ones.