Sahastra Mahabhadra Tap is generally understood in Jain practice as a very high-level tapasya (penance), but it is not one single universally fixed tap in all Jain traditions.

• Sahastra means thousand.
• Mahabhadra means greatly auspicious / highly meritorious.

So the term usually points to a special, intensive austerity or devotional observance done with deep bhakti, self-restraint, and purification of karma. In practice, its vidhi may vary by Śvetāmbara and Digambara traditions, and also by local custom.

In simple words:

• It is a serious Jain tap
• Done for atmashuddhi (purification of the soul)
• With the aim of nirjara and spiritual progress
• The exact method depends on the guru, sampradaya, and local tradition

If you want, I can also explain it in Gujarati/Hindi or give the usual vidhi followed in Jain tradition.