[Guṇḍicā mandira-mārjana is the cleansing (mārjana) of the Guṇḍicā temple (mandira). This festival is performed every year in Jagannātha Purī on the day before the Ratha-yātrā festival in which Lord Jagannātha travels from the main Jagannātha temple, representing Dvārakā, to the Guṇḍicā temple, representing Vṛndāvana. While residing in Jagannātha Purī, Śrī Caitanya Mahāprabhu used to lead all of His associates in cleansing the Guṇḍicā temple on this day.]

We should try to clean our heart. The first thing to clean out is aparādha (offenses to Vaiṣṇavas, to the holy name, to the holy dhāma, and to other living beings), which are represented by huge stones. After that is kuṭināṭī (hypocritical behaviour), jīva-hiṁsana (violence to any living entity), lābha-pūjā (the desire to be worshiped), and pratiṣṭha (the desire for honour and fame); these are represented by dust.
Even after big grasses and stones are cleared away, dust remains, and merely sweeping with your broom will not be able to fully clear away the dust. That dust must be washed with water. In fact, water alone will not suffice. We must take a wet cloth and rub it on that area, after which the area will become clean.
In this way we have so many anarthas, unwanted and unhelpful habits and thoughts, which must all be cleaned out.
Committing aparādhas is considered greater in degree in terms of its inviting severe reactions, and other sins are less in degree. Being a thief or beating a person is considered relatively less in degree, in comparison to vaiṣṇava-aparādha. The results of all other sins can be eradicated, but it is very difficult to become free from the grave reaction to vaiṣṇava-aparādha.
Śrī Caitanya Mahāprabhu has said that if we want to bring Kṛṣṇa (Jagannātha) into our heart, we will have to clean it as He Himself cleaned the temple. Who will clean our hearts? Vaiṣṇava and guru will do so, but we must try to follow their instructions to us. The performance of bhajana will not suffice without taking full advantage of sādhu-saṅga.
It is essential to take care against offenses. Śrīla Bhaktivinoda Ṭhākura has written that merely chanting, remembering, and reading will not suffice in the development of bhakti unless it is accompanied by avoiding vaiṣṇava-aparādha and engaging in any other sinful activity or activity that is unfavourable for bhakti.

―Śrīla Bhaktivedānta Nārāyaṇa Mahārāja