Lilac (Syringa) is a genus of flowering plants containing between 20 and 25 species. Lilacs may be coloured white, blue, purple, pink, or lavender, but the colour commonly referred to as lilac is that of common lilac (Syringa vulgaris). In the Victorian system known as floriography or 'the language of flowers', lilac symbolizes 'the first emotions of love'. This association may have been the basis of the Ivor Novello song 'We'll Gather Lilacs' from the musical romance 'Perchance to Dream' (1945).
Lilacs are mentioned later on in the text when Maxim de Winter is telling the narrator how much he loves Manderley, and asks her whether she likes syringa.
Listen here to 'We'll Gather Lilacs' on Spotify.