A robo-advisor is an online platform that gives you automated investment advice. To be able to use one, you first need to enter some specifics such as risk appetite or the type of investment you are interested in. Based on these inputs, the robo-advisor recommends an investment portfolio tailored to your needs and liking. The recommended portfolio is actively managed by the robo-advisor, which means the elements of the portfolio are changed by the service provider.
Everything is automated, so after signing up you don't have to do anything beyond transferring the funds you want to invest. With robo-advisors you get a high level of convenience but much less control over your portfolio. The service is cheaper than investing in a mutual fund as everything is automated and thus cost-effective.
The biggest robo-advisors in the US are Betterment, Wealthfront and Vanguard. In the UK and Europe, ETFmatic, Nutmeg, and Scalable Capital are the market leaders.