A trial period at the start of employment, typically 6 months, allowing either party to terminate the contract with shorter notice.
While dismissal is easier during probation, it is not a "fire at will" card. You still cannot fire for discriminatory reasons. Crucially, probation reviews must be documented; otherwise, extending probation is legally difficult.
While you don't need the full formal disciplinary process required for long-term staff, you still need to follow "Fair Procedure." You should point out performance issues, give the employee a chance to improve, and warn them their job is at risk. Firing someone on probation without any prior conversation or warning can still lead to claims under Industrial Relations acts or Equality legislation (discrimination claims).
You can only extend probation if the employment contract specifically allows for an extension and the employee agrees (or is notified before the initial period expires). The total probation period, including any extension, should generally not exceed 11 months to avoid the employee accruing one year of service, at which point they gain Unfair Dismissal protection.