उद्यम the state of being active

उद्यमेन हि सिध्यन्ति कार्याणि न मनोरथैः
न हि सुप्तस्य सिंहस्य प्रविशन्ति मुखे मृगाः

उद्यमेन - by being active
हि - only
सिध्यन्ति - fulfil, to be complete, finish
कार्याणि - work
न - not
मनोरथैः - by musings, dreams
न - not
हि - similarly
सुप्तस्य - slept, inactive, dormant
सिंहस्य - lion's
प्रविशन्ति - enter
मुखे - in the mouth
मृगाः - deers/preys

Deers/preys do not(on their own) enter an inactive lion's mouth.
Similarly, work gets completed only by being active, not by musings/dreams.