Active birth positions are positions that you can use during labour and birth to minimise the duration of labour by promoting more effective contractions and assisting your baby to navigate your pelvis and move into an optimal position for birthing. When you are in an upright position, you allow gravity to help the process of birth, and you help the baby to encourage a favourable position for being born.
Given that all of this is happening, the benefit for you is a less painful labour. Women who are free to move around in labour usually report less pain than women who were restricted in the positions they could adopt. This is true both for labour and for birth.
When you are birthing your baby, an upright position such as kneeling or all fours, will help to prevent tears while speeding the birth process. This can also mean that your baby is born in a better condition as a shorted second stage is correlated with better outcomes for babies.
There is no “ideal” position for labour and birth: the key is to have a few ideas of what you might like, try them out on the day / night and go with what feels right at the time.
Some midwives or obstetricians will be reluctant to attend your birth if you are off the bed, citing a bad back or an inability to kneel down. That is fine as the modern delivery beds can accommodate a range of positions such as squatting with a squatting bar, all fours and kneeling.