It represents the martyrdom of St Agatha- not a subject often depicted in art- and perhaps you can see why.
It's part of a memorial to John and Agatha Lewis, Lord and Lady of the Manor of Church Norton. St Agatha is here because she's the woman's name saint. The whole monument looks like this...
St Agatha is sort of tucked round the corner on the right- at right angles to the figure of Agatha Lewis kneeling at her prie-dieu. John Lewis is also supported by a saint- in his case a fairly tame St George. We need to suppose that the empty central panel once contained a religious image- probably a crucifixion scene.
Here's a close-up of John in his armour and page-boy haircut.
And now for the bigger picture. The Lewis monument is one of the leading features of the Church of St Peter- otherwise known as the Chapel of St Wilfrid- at Church Norton. The history is a little complicated. St Wilfrid the Apostle of Sussex built his base- which became a cathedral- on the Manhood peninsular- which is the sticky out bit of land below Chichester which culminates in Selsey Bill. His building has long since disappeared, but the medieval church probably occupies the same site- because it would, wouldn't it? St Peter's church used to be a quite big but- what with the nearby harbour silting up and the population moving away- the decision was taken in 1864 to demolish it- very carefully- and rebuild it in the nearby town of Selsey- leaving the chancel behind to serve as the mortuary chapel for a graveyard that was- and is- still open for burials.
And now a shout out to the Kiplingites on my FL. You probably know the poem Eddi's Service which is appended to the story The Conversion of St Wilfrid in Rewards and Fairies. Well this- supposing the sites coincide- is the nearest thing standing to The Chapel at Manhood's End where we can suppose the action to have taken place.