The canes are the thick, vigorous shoots that arise from the base--or near the base--of the bush. The laterals are the shorter side shoots that come from the canes. Since 'Gertrude Jekyll' is an everblooming shrub rose, the distiction is less important than it would be with some climbers, or the older once-blooming shrubs. On those, the canes don't bloom, but the laterals that grow the next year do.

Thanks very much!

You're welcome, Angela!