Explain to save custom objecto on Room Database with TypeConverter

TypeConverter

    @TypeConverter
    fun stringToListPromo(data: String?): List<PromoEntity?>? {
        if (data == null) {
            return Collections.emptyList()
        }
        val listType: Type = object :
            TypeToken<List<PromoEntity?>?>() {}.type
        return gson.fromJson<List<PromoEntity?>>(data, listType)
    }

    @TypeConverter
    fun listPromoToString(someObjects: List<PromoEntity?>?): String? {
        return gson.toJson(someObjects)
    }