A Self Invested Personal Pension (SIPP) works in a similar way to most other personal pensions but usually with a greater choice of assets that you can invest in, and you choose them yourself or with the help of an adviser.
Like all pensions, the SIPP is a “wrapper” so you are applying the wrapper to the assets like investment funds, non-residential property and so on. That way, the growth/income you make from investments in your SIPP, qualify for the special tax benefits that pensions offer.