When you switch a job to a different company or retired from your company, you will have the opportunity to move your 401k or similar retirement account to an IRA account called Rollover IRA. 

How to move your investments in your retirement account to a Rollover IRA?  The easiest is to move your investments directly to your Rollover IRA with a brokerage company.  But in most cases, it is not possible to do that.  You have to sell your investments in your retirement account with your company, transfer the cash to your Rollover IRA, then buy back the similar investments.  This is called Lateral Move or Sideways Move.

What are similar investments?  For example, you have invested in a fund  called S&P Index Fund Institutional Share in your retirement account, you can easily find the exact fund in your IRA called SPY or VOO or FXAIX which are all S&P Index funds. 

If you have a fund called Large Cap, a fund called Mid-Cap or a fund called Small Cap, I would roll all these funds to Total Stock Market Index Fund called VTI. 

You might have invested in some Sector Funds, then you will have to find the similar Sector Funds in your IRA.  For example, Real Estate Fund, you can find in Fidelity a similar fund called Fidelity Real Estate Index Fund (FSRNX).