येषाम् for those monkeys, गतिः movement, उपरि upward, न सज्जते do not get impeded, अधस्तात् downward, न not, तिर्यक् horizontally, न not, अमिततेजसः very brilliant ones, महत्सु any kind, कर्मसु of task, न सीदन्ति do not care

"Their course cannot be impeded either upward, or downward or right or left. They are indeed very brilliant and would never fail in any task.