Shankara is called ‘Trilokesh’ for his being the master of three worlds. He is supreme controller of three worlds because he creates these three worlds and also maintains them internally and externally by personally entering into them. One should understand the term ‘Three World’ by upper, middle and lower planetary systems viz. heaven, earth and patala region. Only Lord Shankar is capable to  control these regions and their residents. Actually it is the wonderful pastime of Trilokesh that he creates the world and sustains it by his spiritual effulgence, and then desires liberation by merging the world in his inner self. So that he may get relief from the fatigue of rugby of comic creation and devastation. Kashmiri Keshav Jagadhar Bhatt has brilliantly expressed this notion as under—

इत्येवं भगवन्नबन्ध्यमहिमा निर्माय निर्मानुषं
विश्वं विश्वसितं वितत्य तदनु स्फ़ीतैर्विभूति क्रमैः।
संहृत्याथ निजे महिम्नि निखिलं तत्कन्दुकान्दोलन
क्लेशावेशविरामसंभृतसुखं कैवल्यमाकाङ्क्षसि॥

।Trilokesh is not keeping any distance, while maintaining the world, rather he becomes non-different to the world. He is not just material cause like clay, he is not just efficient cause like a pot-maker, he is not just ruler like a king, rather he is combined of these three together. The fivefold actions of God are— birth, retention, destruction, examination and domination. Domination is also of two types— internal and external.  He resides in the heart as Antaryami and  internally controls all individuals, and he externally controls by sanctioning fruits of one’s actions. If  he would had released the reigns, entire cosmos would had been scattered into pieces. Sun, moon, stars, earth and galaxies would collide with each other, resulting in total destruction within seconds. Certainly there is someone who checks their movements, who directs their path, distance and speed. When cosmic annihilation gets essential, then he destroys everything by performance of Tandava Nritya, the dance of devastation. According to Bhaskar Ray—

ऊर्ध्वाधोमध्यभुवाम् अमरासुरमानवप्रकरनिबिडानाम्।
अन्तर्यमनात् त्रिपुटीसाक्षितया शंकर! त्रिलोकीशस्त्वम्॥

।Purport: O Lord Shankar! You are known as Trilokesh as you are witness of trinity and you wander throughout upper, lower and middle planets which are full of demigods, demons and humans respectively.

Lord Trilokesh is master of many trinities namely Pramata, praman and prameya; Gyan, iccha and kriya; karta, karan and karma; Jagrita,swapna and sushupti. Appreciating his leadership and then following his commands in life, is the true form of worship of Trilokesh.