It means “a hole” in your life, in opposition to “ a mountain”, “rock”; often understood as a breather
a dark valley: mental pain, suffering
a flowery valley: a good message
a boggy valley: little troubles with health
walk the valley: a dream come true
see or trek across: if the valley is beautiful and you do not encounter any obstacles, you will experience happy, but poor days; a murky valley heralds losses due to your luxurious lifestyle.