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.