Basics....To make this simple, if you cross a C. sativa 'Panama Red' x C. ruderalis... the two parents C. sativa 'Panama Red' and C. ruderalis are your parental generation of your filial generation 1 or f1 for short. This is your first crosses seeds. This would result in a hybrid f1 that was 100% heterozygous for autoflowers.
When you grow up those f1 seeds, any cross between them will produce f2 seeds or your second filial generation. If you take your f1 seed and cross it back to one of its parents then you have an F1BC1 or some may simply annotate it as BC1 for first back cross.
If you self your f1 seed or any seed for that matter, that is considered S1 seed or seed that is selfed 1 time.
An example of a backcross with an f1 would be: (C. sativa 'Panama Red' x C. ruderalis) x C. sativa 'Panama Red which could also be written as (C. sativa 'Panama Red' x C. ruderalis F1) x C. sativa 'Panama Red' where it is understood that the first plant in the equation is always your seed bearer and the second is your pollen donor. An f1 between an autoflower and a photoperiod should give you 100% heterozygous plants for the autoflower trait if there is no C. ruderalis hidden in the genetics of the photoperiod.
Since your first plant is 100% heterozygous for the autoflower trait and you are crossing to a flower that is 100% homozygous for photoperiod, you can expect 50% of your F1BC1 seed to be homozygous for photoperiod flowers with 50% of your seedlings being heterozygous for the autoflowering trait.
An R1 is similar to an F1, but is simply a cross done with a reversed female rather than regular male pollen. R1 crosses are great as it allows you to cross two female clones of known traits, such as THC concentration, to each other. As those traits are quantitative traits this is a really good way of working those traits forward, but it requires a lot more effort than using a regular male.
Some will annotate a F1BC1 as simply BC or BX. F1BC2 or F1BX2 are also possible. A BC2 or BX2 depending on how you like to annotate simply means you have crossed back wards to an earlier parent in your line twice.
An example of a BC2, BX2 or F1BC2 would be (Cannabis sativa [('Panama Red' x C. ruderalis) x C. sativa 'Panama Red]' x C. sativa 'Panama Red'. We see the parent plant C. sativa 'Panama Red' is introduced twice into the line.