A non-solicitation clause is a clause in a contract which prevents you from directly seeking work from your client’s clients.
This may be included in a contract you’ve been provided by your client.
Non-solicitation clauses generally ask you not to directly contact or try to get work from the end-client that you’ve been working on, via a third-party.
For example, if you’re hired by a design agency to do some work for Acme Inc, the non-solicitation clause would prevent you from emailing Acme Inc direct, during or after your contract with the agency, and asking for them to more work.
This generally protects your clients from losing business to you as a competitor. It can also prevent that client from contacting you directly to get work done, perhaps because they want to avoid paying the agency.
Some non-solicitation clauses may be overly broad in their scope - for example, asking you not to work with ANY end-client which your direct client is currently working with, or has worked with in the past.
Generally, non-solicitation clauses are acceptable, but ensure you consider the scope of the clause, so it does not restrict your ability to work, for example, only including the end-client you’re directly working on, rather than any other business of your client.
Remember, always seek legal advice if you’re unsure what a contract is asking you to agree to.